PARIS (AFP, REUTERS, BLOOMBERG) - Deadly small particle pollution in four out of five nations worldwide exceeded World Health Organisation (WHO)-recommended levels last year despite Covid lockdowns, according to a report released on Tuesday (March 16).

The partial or complete shutdown of transport and industry for months at a time in 2020 reduced average levels of so-called PM2.5 pollution across the world, including in major cities, the IQAir quality report found.

Concentrations of the life-shortening particles - cast off by traffic pollution and burning fossil fuels - dropped 11 per cent in Beijing, 13 per cent in Chicago, 15 per cent in New Delhi, 16 per cent in London, and 16 per cent in Seoul.

Los Angeles, Melbourne and Sao Paulo, all of which were close to major wildfires in 2020, were among the few cities where pollution was worse than in 2019.

The devastating wildfires that torched a record 1.74 million hectares in the United States in 2020 made North America the only region in the world where air quality was worse than during the previous year.

In the US, 38 per cent of cities had levels of fine particulate matter, known as PM2.5, that exceeded World Health Organisation quality standards in 2020, up from 21 per cent the previous year.

In contrast, at least 60 per cent of India's cities were more breathable last year than in 2019, and all of them had cleaner air than in 2018. Chinese cities regularly rank as some of the most polluted, but last year, 86 per cent of them had cleaner air.

"Many parts of the world experienced unprecedented - but short-lived - improvements in air quality in 2020," said Ms Lauri Myllyvirta, lead analyst at the Centre for Research on Energy and Clean Air (CREA) and co-author of the report. "This meant tens of thousands of avoided deaths from air pollution."

However, only 24 of 106 countries monitored met WHO safety guidelines, said the report, based on the world's largest database of ground-level air pollution measurements. China and many South Asian nations experienced PM2.5 pollution several times greater than WHO-recommended thresholds and in some regions, the concentration was six to eight times higher.

Bangladesh, Pakistan, India, Mongolia and Afghanistan averaged annual PM2.5 concentrations between 47 and 77 micrograms per cubic metre (mcg/m3) of air. The UN says PM2.5 density should not top 25 mcg/m3 in any 24-hour period, or 10 mcg/m3 averaged across an entire year.

The most polluted capital cities in the world last year were New Delhi (84 mcg/m3) and Dhaka (77 mcg/m3), with Jakarta, Kathmandu, Islamabad, Hanoi and Beijing all in the top 20.

About half of all European cities exceed WHO's suggested limits.

Air pollution shortens lives worldwide by nearly three years on average, and causes more than eight million premature deaths annually, earlier studies have found.

PM2.5 particles penetrate deep into the lungs and enter the bloodstream. In 2013, the WHO classified it as a cancer-causing agent.

Compared to other causes of premature death, air pollution worldwide kills 19 times more people each year than malaria, nine times more than HIV/AIDS, and three times more than alcohol.

IQAir's analysis suggests that as cities tightened lockdowns last year, pollution tended to improve, but these gains were lost when movement restrictions eased. This indicates 2020's improvements will prove fleeting without significant changes to the energy mix and to human behaviour.

Pollution levels are already returning to pre-pandemic levels, according to satellite data published on Monday by the European Space Agency.

In a related development, the US Environmental Protection Agency said on Monday it will require power plants in a dozen states to cut their smog emissions starting this year as part of an effort to help areas of the country that are downwind of polluting industry.

The EPA estimated the changes to the pollution rule would reduce emissions by 17,000 tonnes beginning in 2021. "The public health and climate benefits are valued annually, on average, at up to US$2.8 billion (S$3.77 billion) each year over the period 2021 to 2040," the agency said.